Jadeite with the Ca-Eskola molecule from an ultra-high pressure metagranodiorite, Dora-Maira Massif, Western Alps

Abstract.

A metagranodiorite from the ultra-high pressure (UHP) Brossasco–Isasca Unit (BIU) of the southern Dora Maira Massif, Western Alps, contains relicts of jadeite developed as a pseudomorph after igneous plagioclase during the Alpine UHP metamorphism. The jadeite is characterised by a slight but significant shortage of total cations. On average, for 6 oxygens, the sum of M2 (Na + Ca) cations is 0.93. Careful algebraic calculations on the jadeite end members indicate that Ca and Al combine to form the calcium-Eskola (CaEs) molecule (0.08–0.17) and minor calcium-Tschermack (CaTs) molecule (≤0.03). This is the first report of jadeite with significant amounts of CaEs component from a natural UHP metagranitoid rock. The finding of CaEs molecule in the metagranodiorite is further evidence that the rock experienced UHP metamorphism during the Alpine metamorphism.